The invention relates to an apparatus for testing the optical qualities and coatings of optical components by determination of the magnitude of light rays or waves disarranged by an imperfection either on the surface or internally of the optical component.
It is important in many modern weapon applications to achieve a high degree of sensitivity to light rays emitted from a target. This is especially true of missiles incorporating infrared detectors utilized for sensing a target and guiding the missile to the area of the target by means of the infrared energy sensed. These missiles normally have a dome, to protect the detector, which is transparent to the frequency of the infrared energy to be detected. Imperfections on the dome surface or interenally to the material of the dome masks the infrared rays passing through the dome to the detector due to scattering or diffusion of the infrared rays. This can cause missile insensitivity to weak target signals created solely by the dome itself and can seriously degrade missile performance.
Another area where imperfections in optical components could have a deleterious effect on over-all system performance is in the field of fire control instruments that utilize optical components such as lenses, prisms, mirrors, reticles, windows, wedges and the like. Any defects in the optical glass utilized in the manufacture of these devices or major scratches on their surfaces could have a harmful effect in that the sensitivity of the instrument could be seriously degraded by scattering of a portion of the light rays passing through the components.
Realization in both industry and government of the effect of the quality of optical components on over-all system performance has caused the establishment of certain required testing techniques and standards to accurately measure the surface roughness of manufactured optical components. The testing technique utilized at present that has wide acceptance throughout the manufacturing industry is a determination of the magnitude of scratches and digs existing on the surface of an optical element. Certain standards have been established over a long period of time that define the quality of the lens or other device tested by the number, length, depth and location of the scratches and digs present on the surface. One method of measuring these defects is by moving a stylus tip across the imperfection and relating the vertical movement of the tip to a predetermined established standard for the particular imperfection found. It can be appreciated that this technique has some major drawbacks when working on optical components where there is a requirement for a supersmooth surface. Certain inherent inadequacies such as size of stylus tip and skill of the test operator may prevent detection of imperfections that would otherwise cause rejection of the optical components. In addition a stylus tip could cause scratches as it is moved across the optical component surface and these scratches could result in destruction of the tested component.
Another inspection technique accepted by the government and optical industry is a comparative test which is accomplished by comparing a standard optical component that has been engraved with standard scratches and standard digs. Optical inspection personnel grade the optical component being inspected by "eyeball" comparison of its imperfections to those standard imperfections on the optical standard. A test such as this causes great variations to exist from manufacturer to manufacturer as well as between different optical inspection personnel since the skill of individual inspectors vary as well as their judgement and estimating ability. In view of this lack in the industry there is a need for an instrument that will give an objective test based on measurements that can characterize optical components objectively in terms of functional parameters in the systems where the optical components are utilized and in terms compatible with presently accepted standards.
The present optical testing apparatus overcomes all of these defects inherent in the present testing techniques discussed above and in addition permits tests of optical performance of components that are not possible to specify using present day standards. Optical components other than glass can be tested and also the instrument can be used to determine the quality of transparent and opaque components in the visible and infrared spectrum. Further the inventive apparatus is capable of measuring surface, surface coating and bulk optical properties objectively without reliance on the skill or judgement of inspection personnel.